This invention relates to the control of power levels of transmitted signals in telecommunication systems, in particular radiocommunication systems.
Good transmit power control methods are important for communication systems having many transmitters that transmit simultaneously to minimize the mutual interference of such transmitters while assuring high system capacity. As an example, such power control is important for communication systems that use Wideband Code Division Multiple Access (W-CDMA). Depending upon the system characteristics, power control in such systems can be important for transmission in the uplink (i.e., for transmissions from a remote terminal to the network), the downlink (i.e., for transmissions from the network to the remote terminal), or both.
To achieve reliable reception of a signal at each remote station, the signal to interference ratio (SIR) of the received signal should exceed a prescribed threshold for each remote station (referred to as a “required signal-to-interference” level or SIRreq. For example, as shown in FIG. 1, consider the case in which three remote stations receive, respectively, three signals on a common W-CDMA communication channel. Each of the signals has a corresponding energy level associated with it, namely energy levels E1, E2 and E3, respectively. Also present on the communication channel is a certain level of noise (N). For the first remote station to properly receive its intended signal, the ratio between E1 and the aggregate levels of E2, E3 and N must be above the first remote station's required signal-to-interference ratio SIRreq.
To improve the SIR of a received signal, the energy of the transmitted signal may be increased, depending on the SIR measured at the receiver. A technique for controlling transmit power in radiocommunication systems in this manner is commonly referred to as a fast power control loop. According to this technique, an initial SIR target is established based upon a desired quality of service (QoS) for a particular connection or service type. For non-orthogonal channels, the actual SIR values measured by a particular remote station or base station can be expressed as:                     SIR        =                              Mean            ⁢                                                   ⁢            power            ⁢                                                   ⁢            of            ⁢                                                   ⁢            received            ⁢                                                   ⁢            signal                                Sum            ⁢                                                   ⁢            of            ⁢                                                   ⁢            the            ⁢                                                   ⁢            power            ⁢                                                   ⁢            of            ⁢                                                   ⁢            all            ⁢                                                   ⁢            interfering            ⁢                                                   ⁢            signals                                              (        1        )            The SIR is measured by the receiver and is used for determining what power control commands to send to the transmitter. For example, if the measured SIR at a remote station is less than the target SIR, a power control command is sent to the base station, directing the base station to increase the transmit power. Conversely, if the measured SIR is greater than the target SIR, the transmitted power control command instructs the base station to decrease the transmit power.
A slow power control loop can then be used to adjust the SIR target value on an ongoing basis. For example, a remote station receiver can measure the quality of the signal received from the base station using, e.g., known bit error rate (BER) or frame error rate (FER) techniques. Based upon the received signal quality, which may fluctuate during the course of a connection between the transmitter and the receiver, the slow power control loop can adjust the SIR target that is used by the fast power control loop in the remote station to control the uplink transmit power. Similar techniques can be used to control the downlink transmit power.
A drawback to this technique for power control is that increasing the energy associated with signals transmitted to one remote station increases the interference associated with signals transmitted to other nearby remote stations. Thus, radio communication systems must strike a balance between the requirements of all remote stations sharing the same common channel. A steady state condition is reached when the SIR requirements for all remote stations within a given radio communication channel are satisfied. Generally speaking, the balanced steady state may be achieved by transmitting to each remote station using power levels which are neither too high nor too low. Transmitting messages at unnecessarily high levels increases interference experienced at each remote station and limits the number of signals which may be successfully communicated on the common channel, i.e., reduces system capacity.
System capacity is becoming more and more important as consumer demand for radiocommunication services increases. The addition of services such as facsimile, e-mail, video, Internet access, etc. and the desire of users to access different types of services at the same time, e.g., videoconferencing, will require efficient use of system resources.
One technique for handling different types of data communication involves providing a different radio bearer, also referred to as a transport channel or transport format combination (TFC), for each service. A radio bearer provides the capability for information transfer over the radio interface and is characterized by an information transfer rate (i.e., bit rate or throughput) and delay requirements, among other attributes. A radio bearer carries either user data or control signals. Typically, a radio bearer is used for a specific service, e.g., speech. A radio bearer may span several physical channels, or multiple radio bearers may share a physical channel, depending on the bandwidth requirements of each radio bearer.
In addition to one or more radio bearers, a user is typically allocated one or more Physical Data Channels (PDCHs) or Dedicated Physical Data Channels (DPDCHs) which carry user bits. A user or terminal is also typically allocated a Physical Control Channel (PCCH) or a Dedicated Physical Control Channel (DPCCH) on which overhead control information is carried to the user, e.g., bit rate information of the associated PDCHs, transmit power control bits and pilot symbols, at a constant bit rate, which can be used to make the SIR measurements used in the fast power control loop process.
The DPCCH is transmitted at a power level needed to ensure proper power control, i.e., to ensure an adequate quality of the received power control bits. In general, there is a minimum quality of service (QoS) which must be met by the DPCCH, to ensure proper power control. To meet the desired QoS, the power level at which the DPCCH is transmitted must reach at least a minimum level. For example, the DPCCH in W-CDMA is typically transmitted with a constant spreading factor and at a power level needed to ensure proper performance of TFCI decoding and feedback of information in the FBI bits.
A minimum QoS must also be met by the DPDCH. However, the power level of the DPDCH also depends on the data rate.
The DPDCHs and DPCCHs are typically transmitted at different power levels, based on their different requirements. Therefore, there is a power offset between the DPDCHs and the DPCCHs.
Power offsets between the uplink DPDCH and DPCCH channels in an exemplary wideband CDMA system are specified in 3rd Generation Partnership Project, Technical Specification Group Radio Access Network, Spreading and Modulation (FDD), 3G TS 25.213, V3.1.0, the disclosure of which is incorporated here by reference. Examples of power control techniques involving such offsets can be found in U.S. patent application Ser. No. 08/874,907, entitled “Multiple Code Channel Power Control in a Radio Communication System”, filed on Jun. 16, 1997, which is incorporated here by reference.
The manner in which these offset values are determined and updated, e.g., as data rates associated with TFCs vary, is not yet defined. In considering how to manage such offsets, it is important to recognize that if the ratio between the transmit power of the DPCCH and the transmit power of the DPDCH is incorrect, then the power control loop which is working on the DPCCH will have the wrong SIR target. This leads either to a degraded performance on the DPDCH, because the Eb/lo ratio (received signal to interference ratio) of the DPDCH is decreased compared with previous slots, or to a good quality on the DPDCH but at an unnecessarily high power level which increases the interference on other channels.
Accordingly, there is a need for a technique for managing power offsets between channels, in particular data channels and control channels, in radiocommunication systems.